1. Field of the Invention
The present invention relates to a signal generator. Such a signal generator is preferably for generating a xe2x80x98solidxe2x80x99 used in a video wipe generator of a vision mixer.
A solid is an electrical signal representing a three dimensional surface of a desired shape. It comprises at least one ramp signal and typically comprises a combination of at least two ramp signals which themselves may be modified. It may also comprise a signal defined by a polar coordinate system representing a curved surface.
2. Description of the Prior Art
Reference will now be made to FIGS. 1 to 3 of the accompanying drawings which show background to the present invention.
FIG. 1 illustrates a known simple wipe between two video sources X and Y.
As the wipe proceeds as indicated by arrow W, video X is replaced across the display by video Y (or vice versa). The effect of a wipe is achieved by mixing the video sources X and Y according to
KX+(1xe2x88x92K)Y
where K is a keying signal. The keying signal K is derived from a xe2x80x98solidxe2x80x99. This will be explained with reference to FIGS. 2 and 3 i.e. a function having a value depending on the h and v co-ordinates within the picture, where v represents line number and h represents pixel position along a line.
FIG. 2 a illustrates a known example of a xe2x80x98solidxe2x80x99 which is a simple ramp. As shown in FIG. 2, a clip level CP is defined. It will be appreciated that over a field or frame, the clip level defines a plane referred to herein as the clip plane, which will be described in more detail with reference to FIG. 4 below. The keying signal K is, in known manner, derived from the solid by applying high gain to the solid and limiting the result, as shown in FIG. 2B. The keying signal has two levels 0 and 1. The transition between the levels occurs where the solid intersects the clip plane CP. The position of intersection is varied, to produce the wipe, by adding an offset to the solid.
FIG. 3 is a schematic block diagram of a wipe generator of a vision mixer comprising a solid generator, a clip element, a gain element, a limiter and a mixer which mixes video sources X and Y in dependence upon the keying signal K.
The solid generator produces a solid, for example a ramp as shown in FIG. 2A. The clip element applies an offset to the ramp to vary the intersection of the ramp with the clip plane CP as shown in FIGS. 2A to 2C. Gain is applied to the offset ramp, in the gain element and the result limited in the limiter to produce the signal K. The amount of gain applied may be varied as shown in FIG. 2B: that varies the slope of the transition between the limit values of the keying signal K.
The mixer mixes the video sources X and Y according to
KX+(1xe2x88x92K)Y.
This is K=1, the output is X, if K=0 the output is Y.
If the gain applied to the solid is unity and the clip offset is zero, the solid and the keying signal are identical
A known wipe pattern is a clock wipe which is so-called because it proceeds as a rotation of a line.
According to one aspect of the present invention, there is provided a signal generator comprising
a first ramp generator which produces a video ramp signal R1 for each pixel of a predetermined set of pixels h where h=0 to n of each of a predetermined set of video lines v where v=0 to m, of a picture, wherein
R1=A1h+B1v+C1
xe2x80x83where A1, B1, and C1 are coefficients,
a second ramp generator which produces a video ramp signal R2 for each pixel of a predetermined set of pixels h where h=0 to n of each of a predetermined set of video lines v where v=0 to m of the picture, wherein
R2=A2h+B2v+C2
xe2x80x83where A2, B2, and C2 are coefficients,
means defining the an angle of rotation,
means for varying A2 and B2 in accordance with the change in the angle of rotation, and
ramp combining means including NAM means for non-additively mixing the first and second ramps.
Such a signal generator produces a rotating wipe pattern simply by varying the coefficients of one (or both) of the ramps and NAMing the ramps together. The two ramps may rotate simultaneously or one of them may be fixed. Two ramps may rotate in opposite directions simultaneously.
Several rotating wipe patterns can be produced simultaneously by providing more ramp generators and suitably NAMing the ramps. For some patterns the ramps to be NAMed may be selected in dependence upon position in the picture, and the selection may be made according to the quadrants of the picture occupied by the combined ramps. Preferably the ramps are controlled to rotate in portions, e.g. quadrants, of the frame to avoid the need for selection.
For some patterns it is necessary to invert the sense (positive or negative) of the NAM function in dependence upon angle of rotation.
According to another aspect of the invention, there is provided a signal generator comprising
a plurality n of ramp generators which produces video ramp signals Rn for each pixel of a predetermined set of pixels h where h=0 to n of each of a predetermined set of video lines v where v=0 to m, of a picture, wherein
Rn=Anh+Bnv+Cn
xe2x80x83where An, Bn, and Cn are coefficients,
means defining the an angle of rotation,
means for varying at least some of the coefficients An and Bn in accordance with the change in the angle of rotation, and
ramp combining means including means for non-additively mixing ramps and means for selecting ramps and/or selecting combined ramps.
In an embodiment of the another aspect the combining means comprises a plurality of combining stages each having first and second inputs for receiving first and second ramps and an output, each stage including Non-Additive Mixing (NAM) means for mixing the ramps and selecting means for selectively coupling the output to the first and second inputs.
In an embodiment of the another aspect each NAM means implements a positive NAM function and a negative NAM function, and comprising means for selecting the positive or negative NAM function for the NAM means.
In an embodiment of the another aspect the NAM function is selected in accordance with the angle of rotation.
An embodiment of the another aspect comprises means for controlling the said selecting means to selectively couple the output of a said selecting means to its first and second inputs in dependence upon the positions in the picture of the ramps at its first and second inputs, and means for indicating the positions of the ramps in the picture.